JPH04180521A - Production of high tensile thick steel plate having high yield strength and high toughness - Google Patents

Abstract

PURPOSE:To form fine bainite and to prevent the formation of coarser crystal grains by subjecting a slab specified in C, Si, Mn, S, Al, Nb, Ti, and N to hot rolling, accelerated cooling and tempering treatment under prescribed conditions. CONSTITUTION:The steel consisting of 0.04 to 0.18% C, 0.1 to 0.5% Si, 0.7 to 2% Mn, <=0.025% S, 0.01 to 0.08% Al, 0.005 to O.035% Nb, 0.005 to 0.03% Ti, 0.0015 to 0.008% N, and <=0.45% Ceq expressed by formula and the balance Fe is smelted. The slab of this steel is heated to >=Ac3 transformation point temp. and is hot-rolled to 50 to 120mm rolling completion thickness at >=30% cumulative draft at (Ac3 transformation point +50 deg.C) or below. The hot rolling is completed at (Ar3 transformation point +130 deg.C) to Ar3 transformation point temp. in the central part of the plate thickness. The steel plate is then subjected to the accelerated cooling down to 600 to 350 deg.C surface temp. at >=0.4 deg.C/sec cooling rate in the central part. The steel plate is thereafter heated to (Ac3 transformation point -10 deg.C) to 500 deg.C and is subjected to the tempering treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is applicable to ships, marine structures, etc. with a yield strength of 4.
The present invention relates to a method for manufacturing a thick-walled high-tensile steel plate with high yield strength and high toughness of 0 kgf/mm2 or more.

(Prior Art) In recent years, as the loading capacity of ships, especially container ships, etc. has increased, high-strength, thick-walled, high-tensile steel plates have been used in their construction to reduce their weight.

However, manufacturing high-strength, thick-walled steel materials requires not only high strength and toughness but also low cost, so there are many difficult points in the manufacturing method.

Conventionally, in the production of high-strength, thick-walled steel plates, only the quenching and tempering method has generally been adopted. For example, JP-A-60-
There is a method of ensuring predetermined characteristics by twice hardening, such as the method described in Japanese Patent No. 169517. Also, JP-A-5
A method has also been proposed in which heating and rolling are repeated, followed by quenching and tempering, as in the method described in Japanese Patent No. 9-80718.

(Problem to be solved by the invention) However, the one- or two-time quenching and tempering treatment methods and heating-rolling methods proposed in the above-mentioned JP-A-60-169517 and JP-A-59-80718 have been proposed. With the method of repeatedly quenching and tempering, it is difficult to impart the required strength to thick steel plates with a thickness of over 50 mm, and an increase in manufacturing costs due to heat treatment costs and bulk cannot be avoided. There's a problem.

The present invention was made in order to solve such problems, and in addition to adjusting the chemical composition, the rolling completion temperature and cooling rate at the center of the thickness of the steel plate are limited, and then heat treatment is performed. The object of the present invention is to provide a method for manufacturing a thick-walled high-tensile steel plate with high yield strength and high toughness.

(Means for Solving the Problems 9) Therefore, in order to solve the above problems, the present inventors
Plate thickness 50 with high yield strength and tensile strength, and excellent toughness
As a result of intensive research on the manufacturing method of thick-walled high-strength steel with a thickness exceeding 120 mm, we added a small amount of Nb to the steel.
The non-recrystallized area during rolling is expanded, and during rolling, the cumulative reduction rate from the lower part of the recrystallized area to the non-recrystallized area is 30% or more, and control i11 rolling is performed to refine the crystal grains and continue rolling. Upon completion of the process, accelerated cooling was immediately performed to generate a fine bainite + fine ferrite structure from fine austenite grains, and it was discovered that controlled rolling and accelerated cooling can impart the strength and toughness improvement effects of controlled rolling and accelerated cooling to thick steel plates. This led to the present invention.

The first invention contains C: 0.04 to 0.18%, Si: 0.
10-0.50%, Mn: 0.70-2.0%, S: 0
．． 025% or less, Al: 0.010~o, oso%
, Nb: 0.005-0.035%, Ti: 0.00
5~0.030%, N:0.0015~0.0080%
Ceq or 0145 containing and defined by the following formula
% or less, a steel piece consisting of the balance Fe and unavoidable impurities is heated to a temperature higher than the Ac3 transformation point, (Ac3 transformation point +
50℃) or less, hot rolling is carried out to a completed rolling thickness of more than 50mm and less than 120mm, and the temperature at the center of the thickness of this steel plate is (Ar3 transformation point + 130℃) ~ Ar3 Complete rolling within the transformation point,
Immediately, the cooling rate at the center of the thickness of the steel plate was 0.4℃/se.
Accelerated cooling is carried out to a temperature range of 600 to 350 °C above the surface temperature, and then (Ac + transformation point -10 °C)
C, which is a method for producing a high yield strength, high toughness, thick wall, high tensile strength steel plate, which is heated to a temperature range of ~500°C and subjected to tempering treatment.
eq(%C+Mn/6+(Cr+Mo10V)15+(
Cu+Ni)/15 The fifth invention is Cu: 0.80% or less, Ni: 1.50% or less, Cr: 0.70% or less,
Mo: 0.50% or less, V: 0.080% or less, Ca:
The method for producing a high-yield strength, high-toughness, thick-walled, high-tensile steel plate according to claim (1), which contains one or more selected from 0.0005 to 0.0030%.

(effect) The present invention will be explained in detail below.

First, the reasons for limiting the chemical components in the present invention will be explained.

C is a necessary and useful element for ensuring the strength of steel, and for this purpose it is necessary to add 0.049ti or more. However, if the added amount exceeds 0.18%, the toughness of the steel will deteriorate significantly and the weldability will also deteriorate. Therefore, the amount of C added is in the range of 0.04 to 0.18%.

Si is an element useful for deoxidizing and strengthening steel, and it is necessary to add at least 0.10% or more, and if it is added in an amount exceeding 0.50%, the toughness deteriorates. Therefore, the amount of Sl added is in the range of 0.10 to 0.50%.

Mn is an element necessary to ensure the strength and toughness of steel and to prevent softening of the weld heat affected zone (HAZ), and for this purpose, it is necessary to add 0.70% or more. but,
If the amount added exceeds 2.0%, the weldability and toughness of the HAZ will decrease. Previously, the amount of Mn added was 0.70 to 2
．． The range is 0%.

S is not a preferable element because it forms A-based inclusions and deteriorates the toughness of the base material and HAZ.

Therefore, the upper limit of S is set to 0.0:'59.

AI is an element necessary for removing steel and refining crystal grains, and for this purpose, it is necessary to add 0.010% or more. However, excessive addition produces Al oxide-based nonmetallic inclusions and deteriorates toughness, so the upper limit of the amount added is set to 0.
．． 080%. Therefore, the amount of AI added is 0゜0
The range is 10 to 0.080%.

In the manufacturing process of the present invention, Nb not only prevents coarsening of austenite grains during heating, refines grains during rolling, and strengthens precipitation during tempering treatment, but also has the effect of expanding non-recrystallized regions in thickness. There are important elements that are indispensable for the production of meat materials, and in order to effectively demonstrate these effects, 0.00
It is necessary to add 5% or more. However, since adding more than 0.035% impairs the toughness of the HAZ, the upper limit should be set at 0.
．． 035%. Therefore, the amount of Nb added is 0.0
The range is 0.05% to 0.035%.

Ti is a strong nitride-forming element.
The fine precipitation of iN is effective in making crystal grains finer and improving the toughness of the HAZ. To get this effect, 0.005
It is necessary to add more than % of HA.
In order to deteriorate the toughness of Z, the upper limit is set to 0.0309. Therefore, the amount of T1 added is 0,005 to 0.0
The range is 30%.

N is an element that reacts with T1 to form TiN and is effective in improving the toughness of the HAZ. For this, 0.0015%
It is necessary to add more than that. However, if it is added in excess of o or ooso%, cracks may occur during the production of steel billets. Therefore, the amount of N added is 0.0015~o, o
The range is oso%.

In addition to the above-mentioned components, in the present invention, the following elements Cu, Ni, Cr, Mo,
One or more selected from a can be added.

Cu is an effective element for solid solution strengthening and precipitation strengthening, or Cu is an element that deteriorates hot workability when added in excess.
The amount of addition shall be 0.80% or less.

Ni has a remarkable effect of improving toughness, and as the plate thickness increases, it contributes to improving hardenability, and it also contributes to improving toughness in the center of the plate thickness. I don't like it sexually. I want to, Ni
The amount of addition shall be 1.50°C or less.

Cr is an element that improves hardenability and is effective in improving the strength of thick-walled materials, or if added in excess, it causes deterioration of weldability.

Therefore, the amount of Cr added should be 0.70% or less.

Mo is an element that increases hardenability and resistance to hardening and softening, and has a significant strength-increasing effect.For the steel type of the present invention, addition of more than 0.50% is not recommended from an economic point of view. Undesirable. Therefore, the amount of Mo added is 0.50%
The following shall apply.

Is ■ an element effective in increasing strength due to precipitation effect?
Adding too much will deteriorate weldability and toughness. Therefore, the amount of addition of (2) should be 0.080% or less.

Ca makes MnS spheroidal, improves shock absorption energy, and indirectly reduces defects in steel materials caused by hydrogen. For thick-walled materials such as those of the present invention, this effect is remarkable, and in order to exhibit this effect, it is necessary to
Attachment 1 of 0.0030" [+ or more is required.

Therefore, the amount of Ca added is 0.0005 to 0.003
09v range.

Ceq (carbon equivalent) is a measure for evaluating the weldability of steel, and since an increase in Ceq will deteriorate weldability, it is desirable that Ceq be as low as possible.

Therefore, in consideration of ensuring high yield strength and not impairing weldability, Ceq is set to 0.45% or less.

Next, the reasons for limiting the hot rolling conditions will be described.

In the present invention, a steel slab melted by a conventional method is heated to a temperature equal to or higher than the Ac3 transformation point. The heating temperature of the steel billet is determined to uniformly austenite the steel billet, prevent coarsening of the austenite grains, make the grains finer during rolling, and especially promote the solid solution of Nb, which is essential for the effect of expanding the non-recrystallized region. This is the temperature required to fully perform the transformation, and below the Ac2 transformation point, the coarse crystal grains remaining during solidification before austenitization cannot be rolled and refined, ensuring the specified strength and toughness. I can't do anything. For this reason, the heat temperature of the steel billet paste D is limited to a temperature equal to or higher than AcJ.

309 under cumulative IE conditions below (AcJ transformation power +50℃)
The reason for setting it to 6 or more is as follows.

Even if rolling is applied at a temperature exceeding (Ac, transformation point +50°C), significant coarsening of austenite grains (γ grains) due to recrystallization cannot be prevented, and high strength and toughness cannot be obtained by rolling. In addition, if the cumulative reduction rate is 30% or more, γ
This is the rolling reduction rate required to refine grains, and the rolling reduction rate is 3.
If it is less than 0%, the γ grains will not be refined enough, making it impossible to secure the desired strength and toughness. I wanted to,
The cumulative reduction rate needs to be 30% or more at a temperature below (Ac3 transformation point + 50°C).

The reason for limiting the completed rolling thickness to more than 50 mm and less than 120 mm is that if the thickness is less than 50 mm, high yield strength steel can be manufactured using the commonly used conventional manufacturing method without applying the method of the present invention. On the other hand, when the plate thickness exceeds 120 mm, it is difficult to reduce the thickness to the center.
Because the γ grains cannot be sufficiently refined, and because it is difficult to maintain the specified cooling rate in the center during cooling, the ferrite crystal grains of the modified BjV become coarser and cannot achieve the specified strength and strength. This is because the only thing that can be achieved is toughness.

The rolling completion temperature is set at the center of the plate thickness (Ar3 transformation point +130
The reason why it is limited to the range of ℃) to Ars transformation point is that if the rolling completion temperature exceeds (Ar3 transformation point ++30℃),
The required strength and toughness cannot be obtained due to insufficient refinement of the γ grains. On the other hand, if the rolling completion temperature is lower than the Ar3 transformation point, the required strength may be obtained, but the rolling This is because the presence of elongated ferrite grains significantly reduces the Charby impact absorption energy.

Next, the reason for limiting the accelerated cooling conditions will be explained.

The important point in the present invention is to obtain a fine heinitic ferrite structure through controlled rolling in the above-mentioned limited temperature range and subsequent accelerated cooling, thereby achieving high yield strength and high toughness. be.

The reason for performing accelerated cooling immediately after rolling is that the finer γ
By cooling the grains immediately after rE rolling, it is possible to obtain a finer heinite ferrite structure. By performing accelerated cooling in a state where the surface temperature of the steel plate immediately after cooling is lower than the center, the quenching hardness of the steel plate surface can be lowered, and the difference in strength between the surface layer and the center of the steel plate can be reduced. Also, the cooling rate at the center of the plate thickness is 0.4"C/s.
If the steel sheet is less than ec, a fine heinite-ferrite structure cannot be obtained, and strength and toughness at the center of the plate thickness cannot be ensured. Therefore, the cooling rate at the center of the plate thickness is 0.4℃
/see or more.

If accelerated cooling is stopped when the steel plate surface temperature exceeds 600℃, there will be insufficient cooling and strength cannot be secured.
If the sheet is stopped in a temperature range of less than 350° C., the difference in strength between the surface layer and the center portion will not be reduced even in the tempering treatment after cooling, resulting in a lack of uniformity in the thickness direction. Therefore, the cooling stop temperature is limited to a range of 600 to 350°C.

The steel plate that has been acceleratedly cooled in this manner is subjected to a tempering treatment in order to provide a predetermined strength and toughness as well as uniformity in the thickness direction. If the baking temperature is higher than (Ac, transformation point = lO ℃), the strength will decrease and it will not be possible to secure the specified strength.
At temperatures below 00°C, the effect of tempering is insufficient, the strength increases, and the toughness deteriorates significantly. I wanted to,
The baking temperature is (Ac + transformation point - 1O℃) ~ 500
Limited to ℃ range.

According to the method of the present invention, it is possible to obtain the most preferable strength and toughness by performing tempering treatment,
By combining plate thickness and cooling conditions, it may be possible to obtain sufficient properties even in as-cooled materials. Further, by forcibly cooling the steel plate by water cooling or the like immediately after the tempering treatment, tempering embrittlement can be prevented and toughness can be improved.

Note that the temperatures of Ac3 transformation point, Ac transformation point, and Ars transformation point are determined by the following equations.

Ac5('C+□908-223.7C+43'a, 5
P+30.55i +37.9V-34,48In-2
3,0Ni Ac, JC')・723+22.03i-14,0NI
n-14,4Ni+23.3CrAr3dC)=910
-3 l0C-80kIn-20Cu-15cr-55
Ni-8ONl.

However, each alloying element is expressed by the content (96).

(Example) Examples of the present invention will be described below.

The test steel plates were made by melting and casting low-carbon, low-alloy steel containing the chemical components shown in Table 1 using a conventional method, and manufacturing the obtained steel pieces under the manufacturing conditions shown in Table 2. 108 m (
1) Finished with steel plate.

Test pieces were taken from these steel plates and subjected to a tensile test and a glossy impact test. The results are also listed in Table 2.

Table 1 shows the chemical components, and Table 2 shows the transformation points, manufacturing conditions, and mechanical properties.

(Left below) No. in Table 2! , 3.5.7.9, 11.13.1
5.17 is the method of the present invention, No. 2.4.6. s, 1
0.12.14.16.18 is a comparative method. Below, N
o. Examples will be explained in order.

No. 1.2 is steel type A, and compared to No. 1 of the present invention method, No. 2 of the comparative method is inferior in tensile properties and Charvy impact properties because the cooling rate of accelerated cooling is slow.

No. 3.4 is steel type B, and compared to No. 13 of the present invention method, No. 4 of the comparative method is inferior in tensile properties and Charvy impact properties due to the lower heating temperature.

No. 5.6 is steel type C, and compared to No. 5 of the present invention method, No. 6 of the comparative method has a low rolling completion temperature and a slow cooling rate of accelerated cooling, so it has poor Charby impact properties. Inferior.

No. 7.8 is a steel type, and as compared to No. 007 of the present invention method, No. 7.8 of the comparative method has a lower stopping temperature of accelerated cooling, so it has inferior Charvy impact properties.

No. 9.10 is steel type E, and compared to No. 19 of the method of the present invention, the ~n In H rate of ψ spinning is lower - and the stop temperature of accelerated cooling is lower, so the tensile properties and Nyaruphi impact are lower. The connection characteristics are poor.

Nos. 11 and 12 are steel type F, and No. 11 of the present invention method.
On the other hand, comparative method No. 12 has low rolling completion temperature and accelerated cooling stop temperature, and is therefore inferior in Charvy impact properties.

No. 13/14 is steel type G, No. of the method of the present invention,
In contrast to No. 13, Comparative Method No. 14 has a high rolling completion temperature and a high accelerated cooling stop temperature, so it is inferior in tensile properties and Charvy impact properties.

No. 15.16 is a steel type H. Compared to No. 15 made using the present invention method, No. 16 made using the comparative method has poor tensile properties due to a small cumulative reduction ratio, and is inferior in tensile properties due to a low baking temperature. Poor impact properties.

No. 17.18 is steel type I, and compared to No. 17 of the present invention method, No. 18 of the comparative method has a lower cumulative reduction rate, a higher rolling completion temperature, and water cooling after tempering treatment. The tensile properties and Charby impact properties are poor.

(Effects of the Invention) As explained above, the method for manufacturing a high-yield strength, high-toughness, thick-walled, high-strength steel plate according to the present invention includes adding a small amount of Nb and performing controlled rolling, followed by immediately accelerated cooling. By doing this, fine bainite is generated, coarsening of crystal grains is prevented, and fine ferrite is generated, thereby providing a thick-walled, high-tensile steel plate with high yield strength and high toughness.

Therefore, the present invention has the excellent effect of easily manufacturing a thick-walled, high-strength steel plate with a yield strength of 40 kgf/mm2 or more and high toughness.

Claims (1)

[Claims] (1) C: 0.04-0.18%, Si: 0.10-0
．． 50%, Mn: 0.70-2.0%, S: 0.025
% or less, Al: 0.010-0.080%, Nb: 0.
005-0.035%, Ti: 0.005-0.030
%, N: 0.0015 to 0.0080%, Ceq defined by the following formula is 0.45% or less, and the balance consists of Fe and unavoidable impurities at a temperature above the Ac_3 transformation point. The cumulative rolling reduction rate below (Ac_3 transformation point +50°C) is 30% or more, and the completed rolling thickness is 5.
The steel plate is hot rolled to a thickness of more than 0 mm and less than 120 mm, and the temperature at the center of the thickness of this steel plate is (Ar_3 transformation point + 130
After rolling is completed within the range of transformation point (°C) to Ar_3, the steel plate is immediately accelerated cooled to a temperature range of 600 to 350°C with a cooling rate of 0.4°C/sec or more at the center of the thickness of the steel plate. , after that, (Ac_1 transformation point -10℃) ~ 50
A method for producing a high-yield strength, high-toughness, thick-walled, high-tensile steel plate, which comprises heating to a temperature range of 0°C and performing a tempering treatment. Ceq (%)=C+Mn/6+(Cr+Mo+V)/5
+(Cu+Ni)/15(2)Cu: 0.80% or less,
Ni: 1.50% or less, Cr: 0.70% or less, Mo:
0.50% or less, V: 0.080% or less, Ca: 0.0
2. The method for producing a high-yield strength, high-toughness, thick-walled, high-tensile steel plate according to claim 1, characterized in that the method contains one or more selected from 0.005 to 0.0030%.